Aluminum alloy, method of casting aluminum alloy, and method of producing aluminum alloy product

ABSTRACT

An aluminum alloy is composed of 15% or more and 7.5% or less by mass of silicon, 0.45% or more and 0.8% or less by mass of magnesium, 0.05% or more and 0.35% or less by mass of chromium, and aluminum, assuming that the total amount of the alloy is 100% by mass.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an aluminum alloy that has excellentcastability, workability, and mechanical characteristics, a method ofcasting the aluminum alloy, and a method of producing a product of thealuminum alloy.

2. Description of the Related Art

Forged products of 6061 alloy aluminum, which has excellent strength andtoughness and high corrosion resistance, are used for wheels, suspensionarms and so on for weight reduction of vehicles. However, since the 6061alloy has poor castability, near-net-shape blanks with complicatedshapes are difficult to obtain and extrusion products are often used asblanks instead. Thus, the production cost tends to increase when partswith complicated shapes are produced. For this reason, casting aluminumalloys such as AC4C alloy and AC4CH alloy are used in some cases. Whennet-shape castings that are formed by casting such a casting aluminumalloy, or near-net-shape blanks that are produced from such a castingaluminum alloy are formed into final shape by forging by takingadvantage of their castability, products with complicated shapes can beproduced at low production cost. However, the above casting aluminumalloys have poor workability, compared to the 6061 alloy.

To solve the above problem, Japanese Patent Application Publication No.9-125181 (JP-A-9-125181) discloses an Al—Si—Mg—Fe based alloy that hasimproved hot forgeability. Also, Japanese Patent Application PublicationNo. 7-109537 (JP-A-7-109537) discloses an Al—Si—Mg—Ti—B based alloy thathas improved mechanical characteristics.

In Al—Si based alloys that are excellent in castability, Si iscrystallized into brittle crystals via eutectic reaction. Thus, themechanical characteristics, especially ductility, of an Al—Si basedalloys always pose a challenge. In this case, to improve the ductilityof Al—Si based alloy, it is necessary to finely crystallize Si crystalswhich are formed as a result of eutectic reaction (which are hereinafterreferred to as “eutectic Si”). Thus, Sr, Na, Sb, and Ca are added singlyor in combination as property improving elements for grain refinement ofeutectic Si. However, while these property improving elements areeffective even when present in an extremely small amount, each elementhas a unique problem such as absorption of gas or reaction withfire-resistant materials. Also, since the improving capacity decreasesas the property improving elements are consumed with time after theaddition, component management often cause trouble. Such propertyimproving elements are also added for grain refinement of eutectic Si inthe above related arts, but it is preferred that grain refinement ofeutectic Si is stably achieved by addition of elements other than theabove property improving elements.

A solute element such as Mg is often added to Al—Si based alloys toobtain mechanical strength, especially proof strength, comparable tothat of the 6061 alloy. However, since improvement in strength of Al—Sibased alloys cannot be achieved only by adjusting the amounts of Si andMg, Cu has been added in combination with Mg for improved strength.However, when Cu is added, Cu compounds may precipitate or crystallizeand decrease the corrosion resistance of the Al—Si based alloys.Especially, the addition of Cu tends to segregation in casting productsand may impair the corrosion resistance thereof. Cu is added to theAl—Si based alloy for improving strength also in the related artdisclosed in JP-A-9-125181 and JP-A-7-109537, but it is desired toimprove the strength of Al—Si based alloys without adding Cu in order tomaintain the corrosion resistance thereof.

SUMMARY OF THE INVENTION

The present invention provides a novel aluminum alloy which hasexcellent castability and workability and has high mechanicalcharacteristics. The present invention also provides a method ofproducing a casting that is formed from the aluminum alloy of thepresent invention and has high mechanical characteristics, and a methodof producing an aluminum alloy product that has high mechanicalcharacteristics which is obtained by machining the casting.

A first aspect of the present invention relates to an aluminum alloythat is composed of 3.5% or more and 7.5% or less by mass of silicon(Si); 0.45% or more and 0.8% or less by mass of magnesium (Mg); 0.05% ormore and 0.35% or less by mass of chromium (Cr); and aluminum (Al).

According to the above configuration, the aluminum alloy having theabove composition has excellent castability and can be cast into acomplicated net or near-net shape.

Also, since the Mg contained in an effective amount in the aluminumalloy contributes to grain refinement of eutectic Si, grain refinementof eutectic Si can be achieved without relying on the property improvingelements such as Sr as described above. As grain refinement of eutecticSi can be achieved, the aluminum alloy exhibits high ductility and hasexcellent workability.

In addition, the strength of the aluminum alloy is improved when itcontains an effective amount of Mg and contains Cr. Therefore, thealuminum alloy has improved strength and excellent corrosion resistanceeven if it is free from Cu.

The aluminum alloy according to this aspect may further containunavoidable impurities. The unavoidable impurities may include iron(Fe), and the content of iron in the aluminum alloy may be 0.3% or lessby mass.

The aluminum alloy according to this aspect may further contain 0.05% ormore and 0.3% or less by mass of titanium (Ti).

The aluminum alloy according to this aspect may be free from copper.

The aluminum alloy according to this aspect may further contain at leastone of 0.003% or more and 0.05% or less by mass of strontium (Sr);0.001% or more and 0.03% or less by mass of sodium (Na); and 0.05% ormore and 0.15% or less by mass of antimony (Sb).

In the aluminum alloy according to this aspect, when the silicon iscrystallized via eutectic reaction, crystallized silicon may have anaverage grain size of 5 um or smaller.

A second aspect of the present invention relates to a casting method ofan aluminum alloy. The casting method includes: pouring molten alloycomprising 3.5% or more and 7.5% or less by mass of silicon; 0.45% ormore and 0.8% or less by mass of magnesium; 0.05% or more and 0.35% orless by mass of chromium; and aluminum; and allowing the molten alloy tocool and solidify.

In the casting method according to this aspect, the alloy may furthercontain unavoidable impurities. The unavoidable impurities may includeiron, and the content of iron in the aluminum alloy may be 0.3% or lessby mass.

In the casting method according to this aspect, the alloy may furthercontain at least one of 0.003% or more and 0.05% or less by mass ofstrontium; 0.001% or more and 0.03% or less by mass of sodium; and 0.05%or more and 0.15% or less by mass of antimony (Sb).

In the casting method according to this aspect, solidification of themolten alloy may be achieved by cooling the molten alloy at a coolingrate of PC/sec or faster.

A third aspect of the present invention relates to a method of producingan aluminum alloy product including performing cold working and/or hotworking on an aluminum alloy casting manufactured by using the castingmethod according to the second aspect.

According to the above configuration, an aluminum alloy casting and analuminum alloy product having excellent castability and workability andhaving high mechanical characteristics can be obtained. In addition,when a solution heat treatment and an aging heat treatment are performedon the aluminum alloy casting or aluminum alloy product, spheroidizationof eutectic Si is promoted and greater ductility develops, and the Mg isprecipitated as magnesium silicide (Mg₂Si) and mechanical strength suchas tensile strength and proof strength improves.

In the method for producing an aluminum alloy product according to thisaspect, the cold working and/or hot working are performed on thealuminum alloy casting at a processing rate that provides a cumulativearea reduction of 30% or more.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further objects, features and advantages of theinvention will become apparent from the following description of exampleembodiments with reference to the accompanying drawings, wherein likenumerals are used to represent like elements and wherein:

FIG. 1 is a photograph that is substituted for a drawing and shows themetallic structure of test sample 4-3 of this embodiment; and

FIG. 2 is a graph that shows the changes in ductility which wereobserved when an casting (blank) formed from a high-strength aluminumalloy according to this embodiment was hot-rolled at different rollingreductions.

DETAILED DESCRIPTION OF EMBODIMENTS

The present inventors have found that the strength of an aluminum alloycontaining silicon can be significantly improved without addition ofcopper (Cu) by adding chromium to the aluminum alloy in addition toadding magnesium in such an amount that the ductility of the aluminumalloy is not adversely affected to improve strength. The presentinventors have also found that magnesium contributes not only toimprovement of the strength of the aluminum alloy but also to grainrefinement of eutectic Si.

Description is hereinafter made of an embodiment to implement thepresent invention.

An aluminum alloy according to this embodiment is an aluminum alloy thathas excellent castability and workability, and contains, assuming thatthe total amount of the alloy is 100% by mass, 3.5% or more and 7.5% orless by mass of silicon (Si), 0.45% or more and 0.8% or less by mass ofmagnesium (Mg), 0.05% or more and 0.35% or less by mass of chromium(Cr), and the remainder of aluminum (Al) and unavoidable impurities.

When the Si content is not within the above range, the aluminum alloyhas low castability. When the Si content is less than 3.5% by mass, theamount of shrinkage during casting increases. Then, casting defects tendto occur and defects such as casting cracks tends to occur in thecasting. The Si content is preferably 4.5% or more by mass, morepreferably 5.5% or more by mass. When the Si content is more than 7.5%by mass, shrinkage defects tend to aggregate in the portions of thecasting that solidify last. In addition, when the Si content is toohigh, crystallization of brittle Si grains increases, and the ductilityand mechanical strength decreases. The Si content is preferably 7% orless by mass, more preferably 6.5% or less by mass.

The content of Mg in the aluminum alloy is 0.45% or more and 0.8% orless by mass, assuming that the total amount of the aluminum alloy is100% by mass.

The Mg undergoes eutectic reaction and contributes to grain refinementof Si crystals in the aluminum alloy. The reason for it is not fullyunderstood, but it is considered that the existence of Mg changes theinterfacial energy between Al and Si, and concentration of Mg occurs atthe growth interface of Si and inhibits the growth of Si crystals. TheMg precipitates as Mg₂Si during a heat treatment step, which isdescribed later, and improves the mechanical strengths such as tensilestrength and proof strength. That is, the aluminum alloy needs tocontain an appropriate amount of Mg to strengthen the α-Al matrix phase(the structure of which is described later). When the Mg content is lessthan 0.45% by mass, the grain refinement of eutectic Si cannot be fullyachieved and the matrix phase does not have sufficient strength. The Mgcontent is preferably 0.5% or more by mass, more preferably 0.55% ormore by mass. When the Mg content is too high, a portion of the Mg donot form a solid solution and remains as Mg compounds even after a heattreatment, which decreases the ductility and toughness of the aluminumalloy. The Mg content is preferably 0.7% or less by mass, morepreferably 0.65% or less by mass.

Cr is contained in an amount of 0.05% or more and 0.35% or less by mass,assuming that the total amount of the aluminum alloy is 100% by mass.The Cr forms solid solution or precipitates as Cr compounds in the α-Almatrix phase and strengthens the matrix phase to improve the mechanicalstrengths such as tensile strength and proof strength. While it isconsidered that the improvement of the aluminum alloy is mainly due toprecipitation of Mg₂Si phase, it is also considered that precipitationof Cr compounds produces a synergistic effect or the existence of Crinfluences the precipitation state of the Mg₂Si phase. Especially, whenthe aluminum alloy is used for machining blanks, Cr is an effectiveelement to prevent recrystallization during machining. When the Crcontent is less than 0.05% by mass, the α-Al matrix phase does not havesufficient strength. The Cr content is preferably 0.1% or more by mass,more preferably 0.12% or more by mass. When the Cr content is more than0.35% by mass, coarse crystals of Cr compounds are formed and theductility and toughness tend to decrease. The Cr content is preferably0.25% or less by mass, more preferably 0.2% or less by mass.

The aluminum alloy of this embodiment may contain various propertyimproving elements as long as the advantages and effects of thisembodiment are not impaired. Specific examples of the property improvingelements includes titanium (Ti), strontium (Sr), sodium (Na), antimony(Sb), and iron (Fe).

Ti is preferably contained in an amount of 0.05% or more by mass and0.3% or less by mass, assuming that the total amount of the aluminumalloy is 100% by mass. The Ti aids in the formation of fine crystalgrains, and provides solid solution strengthening or precipitationstrengthening of the α-Al matrix phase. When the Ti content is 0.05% ormore by mass, sufficiently fine crystal grains are formed andcrystallized grains tends to be isotropically distributed in the alloy.Since columnar crystals tend to grow when the directionality from thecasting mold is strong, Ti may be added in view of the workability inusing the resulting casting as a machining blank. When the Ti content is0.05% or more by mass, the mechanical characteristics are improved sinceshrinkage and so on are distributed finely in the alloy. More preferredTi content is 0.1% or more by mass. When the Ti content is too high,coarse crystals of Ti compounds are formed in the metallic structure andthe mechanical characteristics decrease. Thus, the Ti content ispreferably 0.3% or less by mass, more preferably 0.25% or less by mass,much more preferably 0.2% or less by mass.

Sr, Na, and Sb aid in grain refinement of eutectic Si. While grainrefinement of eutectic Si is achieved by adding Mg in the aluminum alloyof this embodiment, grain refinement of eutectic Si is promoted and themechanical characteristics are further improved when one or more ofthese elements are added. Especially, when a casting of the aluminumalloy of this embodiment is used as a machining blank, it exhibitsexcellent workability. The aluminum alloy of this embodiment preferablycontains at least one of 0.003% or more and 0.05% or less by mass of Sr,0.001% or more and 0.03% or less by mass of Na, and 0.05% or more and0.15% or less by mass of Sb, assuming that the total amount of thealuminum alloy is 100% by mass. When the content of Sr is excessive,fine and coarse eutectic Si are mixed in the resulting alloy andcrystallization of Sr compounds tend to occur. Also, gas absorptionincreases and promotes the formation of cavities which may decrease theductility. Thus, more preferred Sr content is 0.01% or less by mass.When the content of Na is excessive, fine and coarse eutectic Si aremixed in the resulting alloy and the ductility may decrease. Thus, morepreferred Na content is 0.01% or less by mass. When the content of Sb isexcessive, coarse eutectic Si are mixed in the resulting alloy andcrystallization of Sb compounds, which may reduce the ductility, occurs.Thus, more preferred Sb content is 0.12% or less by mass.

Fe is an unavoidable impurity which may be derived from the rawmaterial. Thus, the Fe content is preferably 0.3% or less by mass, or0.2% or less by mass, assuming that the total amount of the aluminumalloy is 100% by mass. When the Fe content is more than 0.3% by mass,crystallization of Fe compounds increases and the ductility decreases.

The aluminum alloy of this embodiment has improved mechanical strengthand is substantially free from copper (Cu), which decreases corrosionresistance as described before. If it is necessary to determine the Cucontent, it should be less than 0.01% by mass. The aluminum alloy ispreferably free from Cu in view of its corrosion resistance.

Manganese (Mn) is generally used to prevent recrystallization ofaluminum alloys. However, the aluminum alloy of this embodiment may befree from Mn since it contains Cr. This is because Mn decreases theamount of Si in solid solution in the α-Al matrix phase. Boron (B) isgenerally used in combination with Ti as an additive element whichcontributes to grain refinement of metallic structure. However, thealuminum alloy of this embodiment may be free from B since it forms TiB,which decreases machinability.

The aluminum alloy of this embodiment has a metallic structure that iscomposed of α-Al matrix phase, and crystallized phase that contains fineeutectic Si crystallized in a network structure that surrounds thematrix phase. The crystallized phase contains crystallized Fe compoundsand so on in addition to the eutectic Si. The matrix phase containsprecipitated compound grains (such as precipitated grains of Mgcompounds and Cr compounds) in addition to the alloy elements (Si, Mg,Cr, Ti, etc.) in a state of solid solution. The eutectic Si contained inthe crystallized phase preferably has an average grain size of 5 μm orsmaller, more preferably 4 μm or smaller, much more preferably 3.5 μm orsmaller. The average grain size of the eutectic Si is the arithmeticaverage of the values of the maximum length (maximum diameter) of aplurality of eutectic Si that is measured by image analysis of amicroscope image obtained by metallographic observation under an opticalmicroscope.

A method of producing a casting of the aluminum alloy of this embodimentis described below. The method of producing a casting of the aluminumalloy of this embodiment essentially includes a pouring step and asolidifying step.

The pouring step is a step of pouring molten alloy that is composed of,assuming that the total amount of the alloy is 100% by mass, 3.5% ormore and 7.5% or less by mass of silicon (Si), 0.45% or more and 0.8% orless by mass of magnesium (Mg), 0.05% or more and 0.35% or less by massof chromium (Cr), and the remainder of aluminum (Al) and unavoidableimpurities into a casting mold. The method of producing a casting of thealuminum alloy of this embodiment is not limited to typical gravitycasting and pressurized casting, and may be die-casting. The castingmold for use in the casting may be of any type such as a sand mold ormetal mold.

The solidifying step is a step of cooling the molten alloy after thepouring step to solidify it. Grain refinement of eutectic Si can beachieved by properly selecting the material and wall thickness of thecasting mold, the dimensions of the casting (or dimensions of the moldcavity of the casting mold), the cooling method and so on to increasethe cooling rate (solidification rate). For example, the average grainsize of eutectic Si can be reduced by selecting a cooling rate of, forexample, 1° C./see or higher, preferably 5° C./sec or higher.

The method preferably further includes a heat treatment step ofsubjecting the aluminum alloy after the solidifying step to a solutionheat treatment and/or an aging heat treatment. The heat treatment steppromotes the spheroidization of the eutectic Si and improves theductility of the aluminum alloy after the solidifying step.

Here, the solution heat treatment is a heat treatment in which thealuminum alloy is maintained at a high temperature and then cooledrapidly to form super-saturated solid solution. The aging heat treatmentis a heat treatment in which the aluminum alloy is heated and maintainedat a relatively low temperature to cause the elements in thesuper-saturated solid solution to precipitate in order to impart thealuminum alloy with a suitable degree of hardness. By these heattreatments, fine precipitates are uniformly distributed and the eutecticSi are spheroidized, whereby an aluminum alloy having highly balancedstrength, ductility and toughness can be obtained. The conditions of theheat treatments may be selected based on the composition and requiredproperties and so on of the casting. For example, the casting may beheated and maintained at 450° C. to 550° C. for 0.5 to 10 hours and thencooled rapidly in the solution heat treatment process. The heatingtemperature and retention time are preferably 490° C. to 535° C. and 0.5to 3 hours, respectively, for a good balance between cost andproperties. The casting may be heated and maintained at 140° C. to 250°C. for 1 to 20 hours in the aging heat treatment process. The heatingtemperature and retention time are preferably 160 to 200° C. and 1 to 5hours, respectively, for a good balance between cost and properties.

An aluminum alloy product is obtained by subjecting the aluminum castingthat is obtained by the above procedure to a processing step. That is,the method of producing an aluminum alloy product of this embodimentessentially include a pouring step and a solidifying step as describedabove and a processing step.

The pouring step and the solidifying step are the same as describedabove. The processing step involves cold-working and/or hot-working thealuminum alloy casting after the solidifying step to obtain an aluminumalloy product. The method cold working and/or hot working is notparticularly limited. For example, the cold working and/or hot workingmay be by forging (extend forging, swaging, etc.), rolling, spinning orthe like. The cold working and/or hot working may be either performedonce or repeated twice or more. Either cold working or hot working maybe performed, or cold working may be performed after hot working.

The processing step is preferably a step in which the aluminum alloycasting is processed at a processing rate that provides a cumulativearea reduction of 30% or more, preferably 50% or more. When processingis performed twice or more, it is preferred that the cumulative areareduction after all the stages of processing is 30% or more, preferably50% or more. By increasing the processing rate, the cast structure isbroken up and finer eutectic Si are formed and uniformly dispersed inthe metallic structure. As a result, an aluminum alloy product that hashigh ductility can be achieved.

The method of producing an aluminum alloy product of this embodimentpreferably further includes a heat treatment step of subjecting thealuminum alloy product after the processing step to a solution heattreatment and an aging heat treatment. The heat treatment step is thesame as described before.

A homogenizing treatment may be performed on the aluminum alloy castingbefore the processing step as needed. The homogenizing treatment is atreatment for incorporation of crystallized phase that is notincorporated in the solid solution and spheroidization of thecrystallized phase, and improves the workability in the processing stepthereafter. As the homogenizing treatment, the aluminum alloy castingmay be heated and maintained at 450° C. to 550° C. for 0.5 to 10 hours.Cooling after the heating is not particularly limited. The heatingtemperature and retention time are preferably 490° C. to 535° C. and 0.5to 3 hours, respectively, for good balance between cost and properties.

The aluminum alloy of this embodiment is suitably used for a castingproduct or forged product which is required to have high strength andcorrosion resistance or a material (such an ingot) from which they areformed. Examples of such products include suspension systems ofvehicles. Examples of suspension systems include upper arm, lower arm,knuckle, axle carrier, disk wheel, and cross member. When the aluminumalloy of this embodiment is applied to these members, significant weightreduction and performance improvement of the vehicles can be achieved.

Examples of the aluminum alloy, the method of producing an aluminumalloy casting and the method of producing an aluminum alloy product ofthis embodiment are described in further detail.

As Test Example 1, test samples 1-1 to 1-9 composed of aluminum alloyshaving different compositions as shown in Table 1 were prepared andtheir mechanical characteristics were evaluated.

In the pouring step and solidifying step, the ingredients were mixed toobtain different alloy compositions, and each mixture was melted toprepare molten alloy, which was then pored into a copper mold withcavity dimensions of 80 mm×70 mm×15 mm and was allowed to cool andsolidify to obtain an aluminum alloy casting.

In the heat treatment step, a heat treatment designated as T6 wasperformed on the obtained castings. In the T6 heat treatment, thecastings were subjected to a solution treatment at 535° C. for 1 hourand then quenched into warm water at 50° C., followed by an aging heattreatment at 170° C. for 3 hours, thereby obtaining castings as testsamples 1-1 to 1-9.

The tensile strength, proof strength and ductility of the test samples1-1 to 1-9 were evaluated. A flat plate tensile test piece with athickness of 3 mm was obtained from a thick central portion of each ofthe castings obtained by the above procedure. The tensile test wasperformed at a cross head speed of 0.3 mm/min using an autographmanufactured by Shimazu Corporation. The 0.2% proof strength wasobtained from a stress-strain curve calculated from the displacement andload measured with a video extensometer. The tensile test was done atroom temperature. The results are summarized in Table 2.

TABLE 1 Composition [% by mass] Test sample No. Si Cu Mg Fe Ti Cr Al 1-13.5 <0.01 0.60 0.17 0.10 0.25 bal. 1-2 5.6 <0.01 0.69 0.18 0.08 0.05bal. 1-3 6.9 <0.01 0.69 0.14 0.15 0.16 bal. 1-4 7.5 <0.01 0.59 0.14 0.020.19 bal. 1-5 6.9 <0.01 0.45 0.14 0.08 0.10 bal. 1-6 9.0 <0.01 0.45 0.150.13 0.12 bal. 1-7 6.9 <0.01 0.89 0.17 0.12 0.11 bal. 1-8 7.3 <0.01 0.590.17 0.12 0.01 bal. 1-9 6.9 <0.01 0.35 0.17 0.12 <0.01 bal.

TABLE 2 Tensile strength 0.2% Proof strength Test sample No. [MPa] [MPa]Elongation [%] 1-1 320 282 10 1-2 332 281 9.2 1-3 333 292 8.2 1-4 336290 8.3 1-5 325 280 8.6 1-6 327 280 5.1 1-7 337 295 5.6 1-8 317 256 8.91-9 285 240 5.3 (AC4CH)

As shown in Table 1 and Table 2, the test samples 1-1 to 1-5, which hada composition within the composition range of the aluminum alloy of thisembodiment (Si: 3.5 to 7.5%, Mg: 0.45 to 0.8%, Cr: 0.05 to 0.35%), had atensile strength of 320 MPa or more, a 0.2% proof strength of 280 MPa ormore, and an elongation of 8.0% or more, which indicates that thealuminum alloy casting had both high mechanical strength and highductility.

In comparison, the test samples 1-6 to 1-9 were not satisfactory interms of mechanical strength and/or ductility. The test sample 1-6,which contained an excessive amount of Si, had a much lower elongationthan the test samples 1-1 to 1-5. The test sample 1-7, which containedan excessive amount of Mg, also had a lower elongation than the testsamples 1-1 to 1-5. The test sample 1-8, which was substantially freefrom Cr, was satisfactory in terms of elongation but deficient inmechanical strength (tensile strength and proof strength). The testsample 1-9 was an Al—Si—Mg based casting alloy (AC4CH) provided in JIS.The test sample 1-9 was not satisfactory in terms of both mechanicalstrength and ductility as compared to the test samples 1-1 to 1-5.

The test samples 1-1 to 1-5 had almost no defect in the castings andhigh castability, but the test sample 1-1, which had a Si content of3.5% by mass, was inferior in castability to the test samples 1-2 to1-5, which had a Si content of 5.6% or more by mass.

As Test Example 2, test samples 2-1 to 2-4 that were composed ofaluminum alloys having different compositions as shown in Table 3 wereprepared, and their mechanical characteristics were evaluated.

The ingredients were mixed to obtain different alloy compositions, andeach mixture was melted to prepare molten alloy, which was then poredinto a copper, mold with cavity dimensions of 80 mm×70 mm×15 mm and wasallowed to cool and solidify to obtain an aluminum alloy casting.

A plate-shaped blank with dimensions of 70 mm×15 mm×15 mm was cut fromeach of the obtained castings, and its surfaces were wet-polished up tothe grit #600. Then, the plate-shaped blanks were hot-rolled. Theplate-shaped blanks were heated by maintaining them in an electricfurnace at 380° C. for 30 minutes and passed between rolls at roomtemperature. Each of the plate-shaped blanks was passed between rollsseven times in total to obtain aluminum alloy products. Adjustment wasmade so that the final rolling reduction rate after the seven passes ofrolling was approximately 65%.

The obtained products were subjected to the same T6 heat treatment as inthe test example 1, to obtain test samples 2-1 to 2-4.

The tensile strength, proof strength, ductility, and hardness of thetest samples 2-1 to 2-4 were evaluated. The tensile strength, proofstrength, and elongation were measured in the same manner as in TestExample 1. The hardness was measured using a Vickers hardness testerafter a load of 5 kg was applied to a thick central portion of each testsample for 25 seconds. The results are summarized in Table 4. In Table4, the tensile strength, proof strength, ductility, and hardness of aforging alloy 6061 (JIS) quoted from “Aluminum Handbook” are also shownfor comparison.

TABLE 3 Composition [% by mass] Test sample No. Si Cu Mg Fe Ti Cr Sr Al2-1 6.9 <0.01 0.59 0.15 0.15 0.16 0.016 bal. 2-2 7.4 <0.01 0.58 0.140.05 0.14 — bal. 2-3 6.9 <0.01 0.89 0.17 0.12 0.11 — bal. 2-4 6.9 <0.010.35 0.12 0.12 <0.01 — bal.

TABLE 4 Test Tensile strength 0.2% Proof Elongation Hardness sample No.[MPa] strength [MPa] [%] (HV) 2-1 367 317 14.3 128 2-2 362 319 14.0 1292-3 360 314 11.5 129 2-4 310 248 17.0 107 (AC4CH) 6061 315 275 12.0 120(Published value)

As shown in Table 3 and Table 4, the test samples 2-1 and 2-2, which hada composition within the composition range of the aluminum alloy of thisembodiment, had a tensile strength of 360 MPa or more, a 0.2% proofstrength of 310 MPa or more, and an elongation of 14% or more, whichindicates that the aluminum alloy product had highly balanced strengthand ductility. Also, since the aluminum alloy products were processed ata cumulative area reduction of approximately 65% or more, the tensilestrength, proof strength, and ductility were all improved compared tothe castings of Test Example 1 (which were not subjected to anyprocessing).

The test sample 2-3, which contained an excessive amount of Mg,exhibited high tensile strength, proof strength, and hardness values.However, the test sample 2-3 exhibited a lower elongation value than the6061 alloy and did not show significant improvement in ductility inspite of the fact that it was processed at a high processing rate. Thetest sample 2-4 was the same in composition as AC4CH as in the case withthe test sample 1-9 and had excellent ductility, but exhibited lowertensile strength and proof strength values than the 6061 alloy.

As Test Example 3, test samples 3-1 to 3-5 composed of aluminum alloyshaving different compositions as shown in Table 5 were prepared in thesame manner as in Test Example 1, and evaluation was conducted to showhow the hardness of castings depends on the Cr content.

The hardness of the test samples 3-1 to 3-5 was measured in the samemanner as in Test Example 2. The hardness was measured at a thickcentral portion of each of the obtained castings. The results aresummarized in Table 5.

TABLE 5 Test sample Composition [% by mass] No. Si Cu Mg Fe Ti Cr AlHardness (HV) 3-1 7.0 <0.01 0.59 0.14 0.15 0.01 bal. 115 3-2 7.0 <0.010.60 0.12 0.14 0.05 bal. 123 3-3 7.0 <0.01 0.61 0.17 0.15 0.10 bal. 1253-4 7.1 <0.01 0.59 0.17 0.15 0.30 bal. 126 3-5 7.0 <0.01 0.58 0.15 0.150.40 bal. 124

As shown in Table 5, the test samples 3-2 to 3-4, which had acomposition within the composition range of the aluminum alloy of thisembodiment, had a HV of 120 or greater. The test sample 3-1, which wassubstantially free from Cr, was deficient in hardness. The test sample3-5, which contained an excessive amount of Cr, exhibited a highhardness value but coarse Cr compounds (not shown) were observed whenits metallic structure was examined under an optical microscope. Thus,it is considered that the test sample 3-5 had low ductility andtoughness.

As Test Example 4, test samples 4-1 to 4-4 that were composed ofaluminum alloys having different compositions as shown in Table 6 wereprepared in the same manner as in Test Example 1, and evaluation wasconducted to show how the average grain size of eutectic Si depends onthe Mg content.

The average grain size of eutectic Si was obtained by observing themetallic structure in a section taken through a thick central portion ofeach casting under an optical microscope. Several fields of view of themetallic structure were photographed at a 200 fold magnification (600μm×480 μm) and a 400 fold magnification (300 μm×240 μm) by an opticalmicroscope. One sample photograph is shown in FIG. 1. The grain size ofeutectic Si was measured on photographs substituted for drawings as FIG.1 using image analysis software “Image-Pro.” The maximum lengths(maximum diameters) of eutectic Si grains in the fields of view weremeasured and an average grain size was calculated by obtaining thearithmetic average thereof. The result is summarized in Table 6.

TABLE 6 Test Composition [% by mass] Size of sample No. Si Mg Fe Ti CrAl eutectic Si [μm] 4-1 7.0 0.30 0.14 0.15 0.15 bal. 6.0 4-2 7.0 0.460.14 0.15 0.15 bal. 3.1 4-3 6.8 0.70 0.17 0.15 0.14 bal. 2.6 4-4 7.10.89 0.17 0.15 0.15 bal. 2.5

As shown in Table 6, in the test samples 4-2 and 4-3, which had acomposition within the composition range of the aluminum alloy of thisembodiment, the average grain size of eutectic Si was as small asapproximately 3 μm. In the test sample 4-1, which had a low Mg content,the average grain size of eutectic Si was as large as 6 μm. In the testsample 4-4, which contained an excessive amount of Mg, the average grainsize of eutectic Si was as small as 2.5 μm, but metallographicobservation reveals that Mg compounds, which did not incorporated intothe solid solution by the heat treatment, were present in the metallicstructure. Thus, it is considered that the ductility of the test sample4-4 was low as in the case with the test sample 1-7 (Table 2).

As Test Example 5, test samples 5-1 to 5-8 that were composed ofaluminum alloys having different compositions as shown in Table 7 wereprepared, and evaluation was conducted to show how the average grainsize of eutectic Si depends on the cooling rate.

The ingredients mixed to obtain different alloy compositions, and eachmixture was melted to prepare molten alloy, which was pored into acasting mold with cavity dimensions of 80 mm×70 mm×T mm in wallthickness and was allowed to cool and solidify to obtain an aluminumalloy casting. The cooling rate was changed by using copper and silicasand shell molds with different wall thicknesses T (15 mm, 22 mm, and 44mm). The cooling rate for each test sample (actual value measured in acentral portion of the casting) is shown in Table 7.

The casting were subjected to the T6 heat treatment in the same manneras in Test Example 1, thereby obtaining castings as test samples 5-1 to5-8.

The average grain size of eutectic Si was obtained by observing themetallic structure in a section taken through a thick central portion ofeach casting under an optical microscope. The average grain size of theeutectic Si was obtained in the same manner as in Test Example 4. Theresults are summarized in Table 7.

TABLE 7 Test Cooling Size of sample Composition [% by mass] rateeutectic No. Si Mg Fe Ti Cr Al [° C./Sec] Si [um] 5-1 7.0 0.46 0.15 0.150.15 bal. 0.27 8.9 5-2 7.0 0.46 0.15 0.15 0.15 bal. 1.0 3.3 5-3 7.0 0.460.15 0.15 0.15 bal. 5.3 2.6 5-4 7.0 0.46 0.15 0.15 0.15 bal. 8.5 2.7 5-57.0 0.30 0.14 0.15 0.15 bal. 0.27 11 5-6 7.0 0.30 0.14 0.15 0.15 bal.1.0 6.8 5-7 7.0 0.30 0.14 0.15 0.15 bal. 5.3 5.9 5-8 7.0 0.30 0.14 0.150.15 bal. 8.5 6.0

As shown in Table 7, in the test samples 5-1 to 5-4, which had acomposition within the composition range of the aluminum alloy of thisembodiment, the average grain size of eutectic Si was smaller than 9 μm,Above all, the average grain size of eutectic Si was smaller than 5 μmfor the test samples, which were cooled at a cooling rate of 1° C./secor higher, and 3 μm or smaller for the test samples, which were cooledat a cooling rate of 5° C./sec or higher. The average grain size ofeutectic Si in the test samples 5-5 to 5-8, which had a low Mg content,exceeded 5 μm even if the cooling rate was 1° C./sec or higher, or even5° C./sec or higher.

As Test Example 6, a test sample 6-1 that was composed of aluminumalloys having a compositions shown in Table 8 was prepared, andevaluation was conducted to show how the ductility depends on therolling reduction in the rolling process.

The ingredients mixed to obtain the alloy composition shown in Table 8were melted to prepare molten alloy, which was then pored into a coppermold with cavity dimensions of 80 mm×70 mm×15 mm and was allowed to cooland solidify to obtain an aluminum alloy casting.

Five plate-shaped blanks with dimensions of 70 mm×15 mm×15 mm were cutfrom the obtained casting, and their surfaces were wet-polished up togrit #600. Then, the plate-shaped blanks were hot-rolled. Theplate-shaped blanks were heated by maintaining them in an electricfurnace at 380° C. for 30 minutes and passed between rolls at roomtemperature. The rolling was performed on the five plate-shaped blanksin such a manner that the final rolling reduction rates of the fiveplate-shaped blanks were 0% (not processed), 20%, 30%, 40%, and 65%,respectively.

The T6 heat treatment was performed in the same manner as in TestExample 1.

A tensile test was conducted on the test samples after the heattreatment in the same manner as in Test Example 1 to measure theelongation. The result is shown in FIG. 2.

TABLE 8 Test sample Composition [% by mass] No. Si Mg Fe Ti Cr Al 6-16.9 0.59 0.15 0.15 0.16 bal.

When castings (blanks) having a composition within the composition,range of the aluminum alloy of this embodiment were reduced in thicknessby 30% or more by rolling, the ductility was improved. The test samplesprocessed at a rolling reduction of 40% or more exhibited an elongationof approximately 14%. That is, it was found that the ductility of thealuminum alloy of this embodiment can be significantly improved when itis processed at a cumulative area reduction of 30% or more, and that amore desirable result can be obtained when the cumulative area reductionis 40% or higher and 65% or lower.

The aluminum alloy of this embodiment is suitable for castings ormachining blanks that have complicated shapes. In this specification,aluminum alloy castings include near-net-shape machining blanks as wellas net-shape castings.

While some embodiments of the invention have been illustrated above, itis to be understood that the invention is not limited to details of theillustrated embodiments, but may be embodied with various changes,modifications or improvements, which may occur to those skilled in theart, without departing from the spirit and scope of the invention.

1. A casting aluminum alloy comprising: 5.6% or more and 7.5% or less bymass of silicon; 0.45% or more and 0.8% or less by mass of magnesium;0.05% or more and 0.35% or less by mass of chromium; and aluminum. 2.The aluminum alloy according to claim 1, further comprising unavoidableimpurities.
 3. The aluminum alloy according to claim 2, wherein theunavoidable impurities comprises iron, and the content of iron in thealuminum alloy is 0.3% or less by mass.
 4. The aluminum alloy accordingto claim 1, further comprising 0.05% or more and 0.3% or less by mass oftitanium.
 5. The aluminum alloy according to claim 1, the aluminum alloybeing free from copper.
 6. The aluminum alloy according to claim 1,further comprising: at least one of 0.003% or more and 0.05% or less bymass of strontium; 0.001% or more and 0.03% or less by mass of sodium;and 0.05% or more and 0.15% or less by mass of antimony.
 7. The aluminumalloy according to claim 1, wherein when the silicon is crystallized viaeutectic reaction, crystallized silicon has an average grain size of 5μm or smaller.
 8. A casting method of an aluminum alloy, comprising:pouring molten alloy comprising 5.6% or more and 7.5% or less by mass ofsilicon; 0.45% or more and 0.8% or less by mass of magnesium; 0.05% ormore and 0.35% or less by mass of chromium; and aluminum; and allowingthe molten alloy to cool and solidify.
 9. The casting method accordingto claim 8, wherein the alloy contains unavoidable impurities.
 10. Thecasting method according to claim 9, wherein the unavoidable impuritiescomprises iron, and the content of iron in the aluminum alloy is 0.3% orless by mass.
 11. The casting method according to claim 8, wherein thealloy further comprises at least one of 0.003% or more and 0.05% or lessby mass of strontium; 0.001% or more and 0.03% or less by mass ofsodium; and 0.05% or more and 0.15% or less by mass of antimony.
 12. Thecasting method according to claim 8, wherein solidification of themolten alloy is achieved by cooling the molten alloy at a cooling rateof 1° C./sec or faster.
 13. The casting method according to claim 8,further comprising performing a solution heat treatment and an agingheat treatment on a solidified aluminum alloy casting.
 14. A method ofproducing an aluminum alloy product, comprising performing cold workingand/or hot working on an aluminum alloy casting manufactured by usingthe casting method according to claim
 8. 15. The method of producing analuminum alloy according to claim 14, wherein the cold working and/orhot working are performed on the aluminum alloy casting at a processingrate that provides a cumulative area reduction of 30% or more.